Polygenic inheritance refers to the kind of inheritance in which the trait is produced from the cumulative effects of many genes in contrast to monogenic inheritance wherein the trait results from the expression of one gene (or one gene pair). In monogenic inheritance, the expression may be predicted according to a phenotypic ratio that follows Mendelian inheritance. Polygenic inheritance is a non-Mendelian form since it is controlled by multiple genes at different loci on different chromosomes expressed together.
In humans, height, weight, and skin color are examples of polygenic inheritance, which does not follow a Mendelian pattern of inheritance. For instance, the height of an adult human is determined by not just a single gene but by more than 400 genes apart from the other non-genetic factors such as environment and nutrition. Since there are several genes are at play in determining a trait, Mendelian inheritance alone may not explicate the phenotype of an organism. A phenotypic ratio where the effect of a single gene can be predicted would therefore not apply to polygenic inheritance.